To decrease the reducing agent ratio of a blast furnace, there is an advantageous technique of using carbon iron composite as a material for the blast furnace to utilize the effect of decreasing the temperature of the thermal reserve zone of the blast furnace due to the use of carbon iron composite (for example, refer to Japanese Unexamined Patent Application Publication No. 2006-28594. Carbon iron composite produced by forming a mixture of coal and iron ore into a formed product and carbonizing the formed product has high reactivity and, hence, promotes reduction of sintered ore. Carbon iron composite also partially contains reduced iron ore and, hence, the temperature of the thermal reserve zone of a blast furnace can be decreased and the reducing agent ratio can be decreased.
A method for operating a blast furnace with carbon iron composite may be performed by mixing ore and carbon iron composite and charging the mixture into the blast furnace as disclosed in JP '594.
Carbon iron composite is characterized by having higher reactivity with CO2 gas as represented by a formula (a) below than conventional metallurgical coke produced by carbonizing coal with a coke oven or the like (hereafter, described as “conventional coke” to distinguish it from carbon iron composite). The reaction in the formula (a) below can be regarded as a reaction of returning CO2 generated through reduction of ore represented by a formula (b) below back to CO gas having reducing power:CO2+C→2CO  (a)FeO+CO→Fe+CO2  (b).
Accordingly, when the reaction of the formula (a) above rapidly occurs in a region where the reaction of the formula (b) above occurs, both of the reactions successively occur to promote reduction of ore.
A region of a blast furnace where CO2 generated from the formula (b) above corresponds to a region where ore is not completely reduced by CO gas, that is, unreduced ore is present.
It is known that ore mainly containing sintered ore in an upper zone of a blast furnace is in the form of independent particles. As reduction proceeds, ore particles having softened and deformed cohere together to form the so-called cohesive zone (for example, refer to The Iron and Steel Institute of Japan, “Tetsu-to-Hagane,” 62, 1976, pages 559-569). Since ore particles having softened and deformed cohere together to form the cohesive zone, the cohesive zone has a small number of voids and has high gas-permeation resistance (for example, refer to The Iron and Steel Institute of Japan, “Tetsu-to-Hagane,” 64, 1978, page S548). This means that reducing gas is less likely to enter the cohesive zone. According to The Iron and Steel Institute of Japan, 62, 1976, reducibility of sintered ore in the cohesive zone is about 65% to 70% and reduction is not completed. Ore not completely reduced in the cohesive zone is, in the state of having a high FeO concentration, melted and dripped, resulting in reduction with solid carbon as represented by the following formula (c):FeO+C→Fe+CO  (c).
This reaction is an endothermic reaction. Thus, a decrease in the reaction rate of the formula (c) above contributes to a decrease in the reducing agent ratio and suppresses variation in furnace heat in a lower zone of a blast furnace, contributing to stable operation.
When carbon iron composite is used in operation of a blast furnace and carbon iron composite is used as a mixture with ore, carbon iron composite is present in the cohesive zone in a temperature range in which the cohesive zone is formed. When reduction of ore is not completed in the cohesive zone as described above, the gasification reaction of carbon iron composite in the cohesive zone becomes slow, which is problematic.
To exhibit the high-reactivity characteristic of carbon iron composite, that is, to achieve rapid transition from CO2 gas to CO gas in the cohesive zone, it is necessary that CO gas is introduced into the cohesive zone so that reduction of unreduced ore proceeds to generate CO2.
Accordingly, it could be helpful to overcome the problem of the existing techniques and provide a method for operating a blast furnace with carbon iron composite in which carbon iron composite is used as a mixture with ore in a blast furnace and slowing of the gasification reaction of carbon iron composite in the cohesive zone can be suppressed.